1. Field of the Invention
The present invention relates to continuous wave radar ranging systems and, in particular, to a signal processor for use therein.
2. Description of the Prior Art
In continuous wave (CW) radar ranging systems, a frequency modulated interrogation signal is transmitted toward a target and is reflected therefrom back to the interrogating unit. The reflected signal is received by the interrogating unit, mixed with a sample of the interrogation signal, and filtered to obtain a difference signal. The finite distance or range between the interrogating unit and the target introduces a round trip delay .tau. between the return signal and the instantaneous interrogation signal sample. Expressed mathematically, EQU .tau. = 2R/C (1)
where R is the range and C is the velocity of light. Hence, since the interrogation signal is frequency modulated with a given modulation waveform, the reflected signal as received at the interrogating unit is delayed in time, and hence shifted in frequency, from the instantaneous interrogation signal by an amount proportional to the range. For example, where a triangular waveform having a total frequency excursion of .DELTA.F and a period of 1/f.sub.m is used to frequency modulate the interrogation signal, the frequency shift or difference frequency f.sub.R, as generated by a suitably filtered mixer, equal to the time derivative of the frequency of the interrogation signal times the round trip time delay, is: EQU f.sub.R = df/dt .times. .tau. = 4.DELTA.F f.sub.m R/C (2)
Thus, the range between the target and the interrogating station may be computed by a measurement of frequency shift f.sub.R.
Conventional processors measure the difference frequency by counting the number of zero crossings in the difference signal that occur within a fixed time interval. More specifically, the difference signal is applied to a counting circuit which develops a signal that is proportional to the rate of zero crossings.
However, the difference signal waveform undergoes periodic phase discontinuities at a rate of twice the frequency (f.sub.m) of the modulation waveform of radar 10. For a description of such phenomenon reference is made to U.S. Pat. No. 3,968,492 issued July 6, 1976 to G. S. Kaplan and to U.S. Pat. No. 3,974,501 issued Aug. 10, 1976 to A. B. Ritzie, both assigned to the assignee of the present invention. It should be appreciated that the phase discontinuities cause the number of zero crossings occuring during a half cycle of the FM waveform to vary, resulting in an ambiguity in the indicated range. Such ambiguity is particularly evident in that a target receding from radar 10 by a distance equal to one quarter (1/4) wavelength of the transmitted signal frequency may, due to the variation in the number of zero crossings during the half FM waveform cycle, appear to advance toward the target.
This phenomenon can be an acute problem in systems operating on targets having complex or changing reflective surfaces. For a more detailed description of such phenomenon reference is made to "Frequency Modulated Radar", D. G. C. Luck, Chapter 4, McGraw-Hill, 1949.
For a description of various methods to minimize quantization of the measured difference frequency due to the phase discontinuities reference is made to the above-cited Kaplan and Ritzie patents.
There are, however, numerous applications in which during the desired range measuring period, the range increases or decreases monotonically, that is, without reversing directions. An example of such monotonic ranging is the measurement of the level of a material in a container as the container is being emptied. Such a measurement is often made in the operation of an iron blast furnace. The furnace is filled with "burden", a mixture of iron ore, coke, and limestone, and as the burden is melted to form molten iron, the level of the burden in the furnace decreases. When the level of the burden in the furnace decreases to a predetermined level, new burden is added through a gate located in the top of the furnace. Molten iron and slag are removed every 15 to 30 minutes from the bottom of the furnace.
Another such application is the monitoring of the drilling rate and depth of a drilling rig bit where during the drilling process, the drill bit continually penetrates deeper into a drilled material.
The present inventor has found that where the measured range varies monotonically quantization error can be substantially reduced by a signal processor of considerably simpler implementation than that required in conventional ranging systems.